These Caramelized Onion Upside Down Tarts are an excellent blend of savory and sweet, with caramelized onions, gooey Swiss cheese, and a glimmer of balsamic glaze all lodged into a golden puff pastry. They’re super simple to throw together but taste like something you’d order at a fancy restaurant. The best part? They’re brief, and elegant, and have that ideal ratio of crisp pastry and rich, melty cheese. Whether you’re serving them as a party appetizer or treating yourself to a light meal…